In the mid-1980s, the concept emerged of a new (third) generation of synchrotron light sources, characterized by unprecedented levels of brightness-and marking a clear departure from the performances of traditional X-ray sources. This project became reality in the mid-1990s; not only were ultra-bright X-ray sources demonstrated feasible, but rapidly became a key factor in X-ray applications to materials science, the life sciences and other disciplines. We briefly review the basic concepts underlying this new generation of X-ray sources, the present performances of working facilities, and some selected examples of their impact on research